The invention relates to macroporous, asymmetric, hydrophilic membranes containing polyaramide and to a process for production of the membranes.
Since the introduction of asymmetric membranes made from cellulose acetate by Loeb and Sourirajan (S. Sourirajan, Reverse Osmosis. Logos Press, London 1970) and made from hydrophobic polymers (U.S. Pat. No. 3,615,024), numerous membranes, in particular for separation of low-molecular-weight and macromolecular components dissolved in water, have been developed and proposed, their structure and suitability have been given in the literature (Desalination, 35 (1980), 5-20) and they have also been successfully tested in industry and for medical purposes.
Many of the membranes described have particularly advantageous properties for achieving specific objectives. As a consequence of their chemical and physical structure, each of the individual membranes can only be optimally suitable for very specific separation problems. This gives rise to the basic need for new membranes for new problems.
EP-A-No. 0,082,433 gives a clear description of the advantages and disadvantages of already known membranes. There are, for example, hydrophilic, asymmetric membranes made from cellulose acetate which have satisfactory anti-absorptive properties, but which leave much to be desired with respect to their thermal and chemical resistance. On the other hand, membranes made from polysulfones or similar polymers have good thermal and chemical resistance, but a pronounced tendency, due to the hydrophobic properties of the polymers employed, to absorb dissolved substances, causing the membrane to become more or less blocked. Although the mixtures of polysulfone and polyvinylpyrrolidone described in EP-A-No. 0,082,433 eliminate the disadvantage caused by the hydrophobicity of the polysulfone, these mixtures are, however, sensitive to exposure to organic solvents.
Hydrophilicity and simultaneous resistance to solvents are found in membranes made from regenerated cellulose. However, these membranes can be hydrolyzed relatively easily in acidic or alkaline media. In addition, they are easily attacked by microorganisms.